Method of connecting workpieces

ABSTRACT

The invention relates to a method of connecting workpieces which is suitable notably for connecting an anode rod ( 5 ) to an end plate ( 6 ) of a rotor sleeve (7) in a rotary anode X-ray tube where on the one hand adequate strength is required and on the other hand an as small as possible cross-section of the anode rod so as to realize a heat barrier. The method is characterized mainly in that the objects are connected to one another by friction welding and that the cross-section is reduced, that is, outside a connecting zone in which the friction weld is situated, in such a manner that the strength of the connecting zone is at least slightly greater than that of the segment of reduced cross-section.

[0001] The invention relates to a method of connecting a first workpieceto a second workpiece, for example, a method of connecting an anode rodto an end plate of a rotor sleeve in a rotary anode X-ray tube.

[0002] In addition to a device for generating an electron beam, rotaryanode X-ray tubes include essential components such as a rotor sleeve,one end of which is closed by an end plate, as well as an anode rod(anode shaft) which is connected thereto and supports an anode disc(rotary anode).

[0003] Said components are made of very different materials. Whereas therotor sleeve is usually made of copper, the end plate consists of aniron nickel cobalt alloy (FeNiCo) or nickel 42, and the anode rod oftenis made of a tungsten zirconium molybdenum alloy (TZM). Generallyspeaking, the anode rod is connected to the end plate by soldering whileusing a high-temperature solder. Said materials, however, have verydifferent melting points so that the soldered joint is often faulty dueto solder gaps, bonding faults etc. However, because the rotor andnotably the anode rod may be subject to severe mechanical loads due toshocks or unbalance of the rotary system, particularly this joint mustsatisfy very severe requirements.

[0004] The problem is accentuated by the fact that the anode rod shouldalso act as a thermal barrier in order to avoid an increased heat flowfrom the anode disc to the rotor sleeve and the rotary system situatedtherebelow, and hence the cross-sectional surface area of this rodshould be as small as possible.

[0005] Because of the foregoing, the rejection rate due to solderedjoints that are faulty to even a small extent only is comparatively highand the strength that can be achieved by way of this joint iscomparatively low.

[0006] DE-OS 29 15 418 discloses a method of connecting workpiecesconsisting of materials having very different melting points; accordingto this method the connection surfaces of the workpieces aremechanically joined by friction welding after which they are permanentlybonded by diffusion welding. This method, however, has a significantdrawback in that it is rather intricate because of the two weldingoperations and that the quality and strength of the joint often are notreproducible.

[0007] Therefore, it is a principal object of the invention to provide amethod of connecting a first workpiece to a second workpiece whichenables these workpieces, for example an anode rod and an end plate in arotary anode X-ray tube, to be connected to one another in areproducible manner and with a high strength.

[0008] This object is achieved by means of a method in accordance withthe invention which includes the following steps:

[0009] friction welding the first workpiece to the second workpiece;

[0010] reducing the cross-section of at least one of the two workpiecesin a segment which lies outside a connection zone in which the frictionweld is situated, that is, in such a manner that the strength of theconnection zone is at least slightly greater than that of the segment ofreduced cross-section.

[0011] Thus, the resistance to fracture is no longer imposed by thefriction weld and any, hardly predictable material influencing that iscaused thereby, but exclusively by the strength of the segment ofreduced cross-section. Because this strength is dependent exclusively onthe type of material used and on the cross-section per se, the strengthof the joint achieved in accordance with the invention can be reliablyreproduced.

[0012] The reduction of the cross-section at the same time reduces thethermal conductivity; this is particularly advantageous for thedescribed application in rotary anode X-ray tubes. The cross-section ismade only as large as absolutely necessary for achieving the strengthrequired for reliable operation. The reduction of the cross-section thusadvantageously combines two different functions.

[0013] A further advantage resides in the fact that the method is alsosuitable for the connection of workpieces of materials having verydifferent melting points, because any influencing of the strength of thefriction weld due to said differences can be taken into account by acorrespondingly greater reduction of the cross-section relative to thatof the connecting zone so as to achieve said object nevertheless.

[0014] Depending on the type, shape and dimensions of the workpieces tobe connected, the method may also be executed in such a manner thatfirst the cross-section of one of the workpieces is reduced andsubsequently friction welding is performed.

[0015] The dependent claims relate to advantageous further embodimentsof the invention.

[0016] The further embodiment in conformity with claim 2 is useful formost materials, because the heat caused by friction welding could weakenthe material in the areas of the workpieces that adjoin the weldedsurfaces.

[0017] The embodiment disclosed in claim 3 is intended to achieve afurther reduction of the thermal conductivity.

[0018] Finally, claim 4 discloses a particularly attractive applicationfor the method in accordance with the invention.

[0019] Further details, features and advantages of the invention willbecome apparent from the following description of a preferred embodimentthat is given with reference to the drawing. Therein;

[0020]FIG. 1 is a diagrammatic longitudinal sectional view of a rotaryanode X-ray tube,

[0021]FIG. 2 is a diagrammatic longitudinal sectional view of a part ofan anode rod and a rotor sleeve during manufacture;

[0022]FIG. 3 is a diagrammatic sectional view as shown in FIG. 2 afterthe manufacture, and

[0023]FIG. 4 shows a detail of FIG. 3.

[0024] A rotary anode X-ray tube as shown in FIG. 1 includes an X-rayenvelope 1 which accommodates a cathode arrangement 3 for generating anelectron beam as well as a facing rotary anode arrangement. The latterarrangement includes essentially a rotary anode disc 4 which is mountedso as to be replaceable at one end of an anode rod 5. The other end ofthe anode rod 5 is connected to an end plate 6 of a cylindrical rotorsleeve 7.

[0025] The electron beam generated by the cathode arrangement 3 isincident on the oblique segment of the anode disc 4 and hence producesX-rays that emanate from the tube 1 via an exit window 2.

[0026] The anode disc 4 is rotated in known manner, via the anode rod 5and the rotor sleeve 7, so as to avoid overheating of the anode disc 4.

[0027] On the one hand the anode rod 5 should transmit the rotary motionand reliably support the anode disc 4 also in the event of shocks orunbalance of the rotary system, while on the other hand it should alsoserve as a heat barrier in order to prevent the previously describedexcessive heat flow in the rotor sleeve and the drive system.

[0028] The anode rod 5 is usually connected to the anode disc 4 byscrewing, so that this connection is not critical in this respect. Inorder to satisfy the described requirements, the anode rod 5 isconnected to the end plate 6 by means of a method in accordance with theinvention.

[0029]FIG. 2 shows the relevant components in a raw state, that is, arotor sleeve 7′ with an end plate 6′ arranged thereon, as well as asolid rod 5′. The rod 5′, the end plate 6′ and the rotor sleeve 7′ arefirst connected to one another by friction welding. The connecting zoneformed by the friction weld between the rod 5′ and the end plate 6′ isdenoted by the reference numeral 81.

[0030] Subsequently the configuration shown in FIG. 3 is formedtherefrom. In conformity with the rendition at a larger scale in FIG. 4,the zone 61 on the end plate 6′, defined by a dashed line, and the zone51, also denoted by a dashed line, on the circumference of the rod 5′are removed by milling or another manner, thus providing the rod with asegment 52 of reduced cross-section.

[0031] It is essential that the removal of the zone 51 at thecircumference of the (worked) rod 5′ does not extend into a connectionzone 82 which includes the connecting zone 81 as well as heatinfluencing zones around this connecting zone 81. The heat influencingzones are the zones in which the properties of the material, notably itsstrength, could be influenced by the heat produced by friction welding.

[0032] The end plate 6′ and the rod 5′ are both also provided with acommon bore 83. Further bores 62, 63 are provided in the end plate 6serve for the mounting of the rotary system.

[0033] As is shown in FIG. 4, the wall thickness of the connecting zone82 is thus greater than the wall thickness of the segment 52 of the(finished) anode rod 5.

[0034] The relative wall thicknesses of the connecting zone 82 and thesegment 52 are then proportioned in such a manner that the followingconditions are satisfied:

[0035] The wall thickness of the segment 52 is only as large asabsolutely necessary to achieve a (still) adequate strength for thetransmission of the rotary motion to the anode disc 4 while taking intoaccount possible shock loads and unbalance of the rotary system. It isthus achieved that the heat flow from the anode disc to the rotor sleeve7 and the drive system remains as small as possible.

[0036] The wall thickness of the connecting zone 82, however, is chosento be so large that the strength of this zone is greater than thestrength of the segment 52. The required wall thickness is dependentnotably on the degree of influencing of the strength of the materialused in the thermal influencing zone arising due to the frictionwelding. For example, in the case of TZM (tungsten zirconiummolybdenum), that is often used for the manufacture of the anode rod thestrength in this thermal influencing zone is halved. This means that thewall thickness in the connecting zone 82 must equal at least twice thewall thickness in the segment 52.

[0037] The joint in conformity with the invention thus satisfies therequirement as regards adequate strength, being determined only by thetype of the material used for the anode rod 5 and by the wall thicknessof the segment 52, as well as the demand for an as low as possiblethermal conductivity, that is, a heat barrier for protecting the rotarysystem against excessive heating.

[0038] As has already been explained, the order of the two steps inconformity with the invention is not essential and can also be reversedif so desirable for technical or other reasons.

Claims:
 1. A method of connecting a first workpiece to a secondworkpiece, which method includes the following steps: friction weldingthe first workpiece (6) to the second workpiece (5); reducing thecross-section of at least one of the two workpieces in a segment (52)which lies outside a connecting zone (82) in which the friction weld issituated, that is, in such a manner that the strength of the connectingzone (82) is at least slightly greater than that of the segment (52) ofreduced cross-section.
 2. A method as claimed in claim 1, characterizedin that the connecting zone (82) includes at least one thermalinfluencing zone formed on one of the workpieces (5, 6) due to thefriction welding.
 3. A method as claimed in claim 1, characterized inthat at least one of the workpieces (6, 5) is provided with a bore (83)so as to reduce the cross-section.
 4. A method as claimed in claim 1,characterized in that the first workpiece (6) is an end plate of a rotorsleeve (7) and the second workpiece (5) is an anode rod for connectionof an anode disc (4) in a rotary anode X-ray tube, the cross-section ofthe segment (52) of the second workpiece (5) being reduced by milling tosuch an extent that an as low as possible thermal conductivity isachieved in conjunction with a strength that is adequate still.
 5. Arotary anode X-ray tube which includes an anode rod that is connected toan end plate of a rotor sleeve, characterized in that a connecting zone(82) in which the anode rod (5) is connected to the end plate (6) has adiameter which is larger than that of a segment (52) of the anode rod(5).
 6. A rotary anode X-ray tube as claimed in claim 5, characterizedin that the joint between the anode rod (5) and the end plate (6) is afriction-welded joint.